I have another type of performance problem with my set up....
Flight Aware stick on an external USB HUB with external Power Supply, latest image on RPi 3B (not plus).
I have run this setup since January, I do weekly statistics with data from FR24 statistics web page.
In february, i noticed a decrease of maximum distances. Initially I had around 250 nm as a weekly average with number of hits >200 nm at around 1000 per day. I have seen a slow decrease of these numbers during the month of February, last week of the month my max distance was down to 200 nm and number of hits >200 nm was almost down to zero. I have restarted RPi many times, also cold start of USB HUB but still poor performance.

2 days ago I switched stick to my Windows PC, installed FR24 feed software and from the very start I am now back to initial performances.

Looks that the RPi environment is giving a few problem with performance.

The FlightAware Pro Sticks (with on-board amp) draw more power that other SDR devices. The RPi needs a power supply rated at 5.1v 2.5 amps. Using a USB hub or cable to connect the Pro Stick to the RPi can affect performance.

I did not observe slow performance drifts yet, it would require a stable analysis over a long period. I was waiting to have a longer overview before doing a feedback, because I also set up a powered USB hub (12V x 4A, with some ferrites cores on all cables). The RPi 3B+ power supply is still a 2.5A. For a week now, I no longer observe the problems of sudden variation and I did not have to disconnect the antenna once again.

I even see improvement in average distance performance. The number of planes above 50nm is now higher than the nearest ones, and their number has greatly increased.

I also note that the aberrant records that completely distorted the statistics have disappeared for a week. I no longer get this daily ultra-short-time data between 300 and 350nm. I don't know if it's related to the HUB and its more stable power supply, but everyone knows what I'm talking about. These bogus data should have been be filtered for a long time right? ;-)

I am also wary of weather conditions that seem to have a significant influence (in addition to holiday-related air traffic). We have just gone from a very good weather to a big depression for a week. So let's wait a little longer before drawing definitive conclusions.